February 3, 2020 at 6:47 pm #7422Bruce RaymondKeymaster
Well, I see that this topic is empty. That seems a shame. I’ll kick it off with a pompous, self-aggrandizing introduction. My name is Bruce Raymond. I live in south Alabama (escaped from the lands of ice and snow). My callsign is ND8I and I came up with the design for the 30m WSPR transmitter TAPR is selling. I’ve also done the 40m WSPR transmitter. Please let me know if you’re interested in other bands.
Let’s face it – the WSPR transmitter design is not rocket science. We start with a Raspberry Pi and add software (written by someone else). The software generates the WSPR signal. Unfortunately, the output power is only ~10 mW and it’s a square wave, so it’s very rich in harmonics. My contribution was to add a stage of amplification and a low pass filter to clean up the output signal.
The transmitters for the 20m, 30m and 40m transmitters all work similarly. The only difference is the corner frequency for the low pass filter. My 30m and 40m transmitters put out ~200 mW. It’s my understanding that the 20m transmitter puts out a bit less (~150 mW?), but I don’t really know – I could be blowing smoke 😉
The software (WsprryPi – available from Github) builds a message and sends out symbols. WSPR encodes its message into 4 different symbols. Think of a symbol as a character (e.g. A, B, C, D) and imagine encoding your message using those 4 symbols (e.g. AAACCABAACDDDBBBB …). Each symbol is mapped onto a different frequency. The frequencies are ~1.5 Hz apart, so a WSPR message has a bandwidth of 4 * 1.5 Hz = ~6 Hz.
I’m happy to help with WSPR problems any way I can, so feel free to write.
I’ve been asked if our WSPR transmitters work with Raspberry Pi 4. It’s a good question. I don’t know about you, but any question I don’t have an answer for is a good question. I bought a Raspberry Pi 4B recently, but I’m still working on discovering whether or not it will work with our WSPR transmitters (my time seems to evaporate). I’ll post my results here when I get them.
73 Bruce/ND8IApril 28, 2020 at 12:05 pm #8408Steven OllmannParticipant
I am excited about WSPR and purchased 20M and 30M modules from TAPR. I purchased two (2) RPi 3 B+ modules and 2.25 AMP power supplies.
The 30M module came up quickly and has been on-the-air for a couple days now.
The 20M module required downloading a GIT hub repository, however the recent Raspbian image (Buster) when compiling complains about incompatibilities in the source that changed in GCC v7.1. Eventually it failed unable to resolve “makedev”.
* Next step was to change OS to Ubuntu, however some issues
* It was getting late but determined to make success, I used the WWoT image and installed on RPi 3.
– I did succeed in bringing it online and properly decode using SDRplay RSPdx.
– When I look at the frequency displayed on WSJT-X, I notice that the main signal is outside of the acceptable range above 1600 Hz.
… I double checked that SDRuno is tuned to the same frequency as WSJT-x and it is.
… WSJT-X is decoding a spur/harmonic that is inband (1400 – 1600 Hz).
* The 20M / RPi is connected to DX-EE dipole I usually use for my Yaesu FT891.
* The 30M / RPi is connected to HF-360 vertical.
* The RSPdx is connected to a cellular antenna inside the house in my office
My question was going to be, is there an incompatibility between WWoT image on the 20M module?
However, when I looked at the output of the 30M module, the main frequency is on the band edge right at 1600 Hz.
This brings me to the software application controlling the output frequency on the RPi’s. Two different RPi’s, two different modules (20M and 30M) and identical images.
I’m thinking that with QRP, I’d want the main energy transmitted to be in receive filter bandwidth.
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